China doll houseplant pruning

My china doll house plant was doing really well, so well that we repotted it (not knowing that they like to be root bound) it seemed to be fine. However, it has grown to about 3ft...it's been in the same pot for about 2 years and now it has lost all of its leaves except on the top, so we have this tall, spindly stem with no branches or leaves and then a clump of branches and leaves on top. My question is how far back can we prune this plant without killing it to propagate a bushier plant again?

Given enough time, and plenty of light, your China doll should grow leaves to match its roots, which it's been busy developing for the last 2 years. Can you set it outside under the trees in the summer? That's what I'd do with it.
While you could simply cut back that long trunk on some types of plants, and it would grow new branches and main stem from the area of the cut, China doll is not so likely to do that. Also,some types of trees you could cut the main stem below the leafy top a certain distance, then stick that into the soil and it would root, producing a plant with a nice trunk and leafy top. Again, China doll doesn't root very well from hard wood. You could, however, try rooting cuttings made from the side branches. Here are a couple of articles that might be useful, now or in the future: https://www.gardeningknowhow.com/houseplants/china-doll/china-doll-plant-propagation.htmhttps://www.gardeningknowhow.com/houseplants/china-doll/prune-china-doll-plant.htm